This invention relates to ink-jet printing techniques, and more particularly to a new method for unidirectional ink-jet printing which reduces mechanical hysteresis and reduces the area of unprintable margins to zero by printing along a spiral locus path.
There exists a method for placing drops of ink on media such that an array of nozzles is swept over the surface of a flat, rectangular media in a rectangular xe2x80x9crasterxe2x80x9d fashion, usually from left to right, and then back from right to left, accompanied by a step-wise motion from top-to-bottom. That method allows the ink-jet nozzle array to essentially xe2x80x9cvisitxe2x80x9d the entire media area one or more times, depending upon the length of the top-to-bottom motion, the length of the left-to-right scan, and the offsets of the sweep start.
During the sweep, ink-jet nozzles are activated at different times to xe2x80x9cshootxe2x80x9d or eject drops of ink, and these drops land upon the media, thereby making text or images visible upon the media. There are a number of undesirable artifacts which accompany the above described process. These errors are generally due to mechanical alignments within and about the mounting of the ink-jet xe2x80x9cheadxe2x80x9d, directionality errors caused by the angles with which the drops are ejected from the nozzles, timing quantization, position sensing, and importantly, mechanical hysteresis.
Hysteresis is an effect that manifests itself by a non-repeatable position trace while moving from left-to-right, and then moving from right-to-left, so that the commanded position has an uncertainty or offset from that of the actual nozzle position. Hysteresis is often the result of friction in a mechanism, accompanied by the normal tolerance of fitted parts, and accentuated by a start-stop motion of the mechanism. Starting friction may be higher than running friction; hence there is a tendency for the heads to move toward one end of their mechanical tolerance at the reversal of the sweep.
All of these effects cause the drop of ink ejected from the nozzle to land on the media with an error in position, and often there are regular visual effects which then appear as a person views the resulting image or text. Some solutions are found by overlapping the xe2x80x9cswathsxe2x80x9d of the sweep, or by only firing the nozzles during one of the scan directions, say from left-to-right, or by making multiple passes over the same region of the media, and choosing a drop-firing pattern which averages the mechanical errors. There are also techniques of automatic calibrations which improve the resulting print quality.
In addition to difficulties of correctly placing ink in position, a usual condition of the mechanism which handles the motion of the media in the vertical direction is that the nozzle array of the ink-jet head cannot move all the way to the edge of the media, thus prohibiting deposits of any ink drops in a margin on both the left and right sides of the media. Additionally, other mechanical constraints prohibit ink drops from being deposited on a top and also a bottom margin of the media.
In accordance with an aspect of the invention, a method is described for ink jet printing with an ink jet nozzle array having a plurality of nozzles at a different radial distance from a center of coordinates. The method comprising a sequence of the following steps;
receiving a print job defining at least one image to be printed during an ink jet printing cycle;
supporting a flat print medium to receive ink droplets ejected by the nozzle array during the ink jet printing cycle;
selectively generating firing pulses to the ink jet nozzle array in dependence on the image to be printed during the ink jet printing cycle;
providing relative rotational and translational motion between the nozzle array and the medium such that a spiral locus centered at a center of rotation coincident with the center of coordinates is defined by the nozzle array relative to the media during the ink jet printing cycle; and
ejecting ink droplets onto the medium in response to the firing pulses during said ink jet printing cycle, including generating firing pulses for respective ones of the nozzle array at different firing rates, wherein nozzles closer to the center of coordinates are fired less frequently than nozzles further away from the center of coordinates.
In accordance with another aspect of the invention, an ink jet printing system is described, which includes an ink jet nozzle array for ejecting ink droplets during an ink jet printing cycle, the ink jet nozzle array including a plurality of nozzles disposed at a different radial distance from a printing center of coordinates. The system includes a controller for receiving data representing an image to be printed and selectively generating nozzle firing pulses dependent on the image. A flat medium is positioned to receive ink droplets ejected by the nozzle array during an ink jet printing cycle. The system further includes apparatus responsive to control signals generated by the controller for providing relative rotational and translational motion between the nozzle array and the medium relative to a center of rotation at the center of coordinates, the motion in synchronism with the firing pulses such that a spiral locus is defined by the nozzle array relative to the media to print the image during an ink jet printing cycle. The controller generates the firing pulses at different rates for respective ones of the nozzle array, wherein nozzles closer to the center of coordinates are fired less frequently than nozzles further away from the center of coordinates.